Heat transfer printing processes involve physically transferring a printed image from one substrate to another. One heat transfer method is melt transfer printing. A design is printed on paper using a waxy ink. The back side is then heated with pressure, while the printed side is in close contact with a final substrate. The ink melts onto the final substrate in the mirror image of the original image.
Another method of transfer printing is film release transfer. The image is printed onto a paper substrate coated with a film of heat tackifiable resin. Upon application of heat and pressure to the back side of the image, the entire film containing the image is transferred to the final substrate. The printing method used in this invention most commonly employs heat activated dyes, such as sublimation dyes. One form of an appropriate transfer process using liquid sublimation inks is described in Hale, et al., U.S. Pat. No. 5,601,023, the teachings of which are incorporated herein by reference. An image is printed onto an intermediate media using heat activated dyes. Heat and pressure are applied to the back side of the media while the image is in close contact with a final substrate. The dyes vaporize and are preferentially diffused into and/or absorbed by the final substrate to form the image on the substrate. The release of the dye during transfer depends on the vapor pressure of the dye and on the rate of diffusion of the dye vapor through the layers of the paper, and the affinity of the dye for the substrate materials such as binders and additives contained in the paper substrate.
Sublimation or heat sensitive ink printing generally requires an intermediate sheet. The intermediate sheet may be paper. The paper may contain an ink acceptor material capable of readily absorbing the ink and allowing droplets to coalesce, yet maintaining high resolution and color density. For effective sublimation transfer to take place, a liquid ink must be readily absorbed into the body of the media while the dye and/or dyes must remain relatively close to the surface of the media. The dyes used in sublimation transfer inks are relatively low in molecular weight and contain minimal active functional groups that inhibit volatility. The dyes are typically chosen from the disperse dye class. Such dyes are substantially insoluble in water or organic solvents. Dispersion of these dyes within the carrier is necessary to produce the ink, and printing of the ink so formed by commonly available digitally driven color printers, such as ink jet printers, requires a dye particle size of less than a few microns.
Images formed from pigments or dyes and subjected to sunlight experience loss of color due to degradation of the colorant material. This photodegradation is caused largely by ultraviolet (UV) radiation from the sunlight. Ultraviolet radiation from artificial light can cause similar changes. The presence of moisture and heat accelerates this degradation. There are two broad classes of UV light stabilizers. One class of compounds is ultraviolet light absorbers (UVA) that act by absorbing harmful UV radiation. An example of a UVA is a benzotriazole, which acts by dissipating UV energy as harmless heat energy. The second class of UV light stabilizers is free radical scavengers. An example of a free radical scavenger is a hindered amine light stabilizer, or HALS. Hindered amine light stabilizers do not absorb UV radiation, but rather scavenge free radicals formed by the breaking of molecular bonds.
Binders or laminates containing UV stabilizers for covering a printed image to protect it from harmful radiation have been used. For example, Yamamoto, et al., U.S. Pat. No. 4,756,963 describes a transfer layer comprised primarily of a thermoplastic resin and containing a UV absorber. The transfer layer is laminated onto the surface of an image. Watanabe, et al., U.S. Pat. No. 5,223,314 describes a cover film with a layer of an "anti-contamination" material composed of a resin which has no affinity for a sublimation-type dyestuff, such as polyethylene, polypropylene, or Teflon, and a UV absorber. Fujimura, et al., U.S. Pat. No. 5,397,763 describes the use of a heat transfer ribbon which may be comprised of a dye dyeable resin, a sublimation dye, and a protective layer, which may contain a UV absorber. An image may then be transferred from this heat transfer sheet via a thermal head to a final substrate, such as paper, in the order such that the dyeable resin is laid down first, followed by the dye, and then the protective layer.
A UV protector has been added to an image or dye receiving layer of a substrate. For example, Tomita, et al., U.S. Pat. No. 5,783,517 describes the use of a copolymer of phenoxypolyethylene glycol-acrylate or -methacrylate and another monomer, along with a UV absorber and/or light stabilizer for increasing the light resistance of images formed thereon. Kushi, et al., U.S. Pat. No. 5,218,019 describes the use of a polyester resin, a cross-linking agent, a releasing agent and a benzotriazole UV stabilizer for forming an image receiving paper or film for sublimation-type thermal dye transfer methods. Sam, et al., U.S. Pat. No. 5,635,441 describes a dye receiving layer on a printing sheet comprised of a butyral resin, a vinylphenolic resin, a polyisocyanate compound, and an optional UV absorber.
Images printed onto an intermediate media using heat activated, or sublimation, dyes may be transferred to synthetic fabric as a final substrate. Fabrics printed in such a manner may be exposed to UV radiation, for example, polyester and polypropylene sportswear. It would be beneficial to be able to provide UV protection to garments printed in such a manner. It is common practice in the textile dyeing field to use UV absorbers and/or hindered amine light stabilizers in the dyeing process to impart some degree of UV protection to the fabric or to the wearer of the fabric. Other methods of imparting this UV protection to synthetic fibers is to incorporate the UV stabilizers into the polymeric backbone during formation of the polymer or to add the UV stabilizer during formation of the fiber.